Fuel cell powder generation method and system

ABSTRACT

A hydrogen-containing gas suitable for use in a fuel cell, especially in a proton exchange membrane fuel cell, is produced from a digestion gas (b) yielded in methane fermentation of organic matter (a), and is then supplied to the fuel cell to generate electricity. A fuel cell power generation method comprises a methane fermentation step (A) for subjecting organic matter to methane fermentation, a pretreatment step (B) for pretreating digestion gas yielded in the methane fermentation step, a hydrogen production step (C) for producing hydrogen-containing gas (c) from the gas which has been pretreated in the pretreatment step, and a fuel cell power generation step (D) for supplying the hydrogen-containing gas produced in the hydrogen production step to a fuel cell to generate electricity. The pretreatment step comprises an alkaline absorption step (B1) for absorbing carbon dioxide or carbon dioxide and hydrogen sulfide contained in the digestion gas yielded in the methane fermentation step into an alkaline absorbent solution.

TECHNICAL FIELD

[0001] The present invention relates to a technology for recoveringchemical energy of organic matter in the form of a hydrogen gas and atechnology for converting hydrogen gas to electric energy with highefficiency, and more particularly to a system for producing a hydrogengas or a hydrogen-containing gas by methane fermentation of organicwastes such as relatively highly concentrated organic waste water ororganic slurry to produce a digestion gas, and then processing of thethus produced digestion gas, and an electric generating system forgenerating electricity by supplying the produced hydrogen gas orhydrogen-containing gas to a fuel cell. The organic wastes include wastewater discharged from food production, wastes from livestock farms, andexcess sludge produced in sewage treatment plants or the like.

[0002] The present invention is also applicable to a methanefermentation gas produced in landfills or the like.

BACKGROUND ART

[0003] In recent years, there has been a growing tendency forenvironmental protection, and therefore an attempt has been made toreform a digestion gas or a biogas yielded in methane fermentation orthe like of organic wastes into a hydrogen-containing gas which is thenutilized to generate electricity in a fuel cell. Conventionally, forexample, a digestion gas is pretreated to remove hydrogen sulfide andcarbon dioxide therefrom for thereby enriching the concentration ofmethane in the gas, and the gas with a higher methane concentration isthen reformed to produce a hydrogen-containing gas having a hydrogencontent of 70 to 80%. The produced hydrogen-containing gas is thensupplied as a fuel gas to the anode of a fuel cell while air is suppliedas an oxidizing gas to the cathode of the fuel cell, thus generatingelectricity.

[0004] In general, a digestion gas produced in methane fermentation ofsludge at a sewage treatment plant is pretreated by a method of wetabsorption in which hydrogen sulfide and carbon dioxide are absorbed andseparated by a large amount of treatment water from the sewage treatmentplant. For a digestion gas produced in methane fermentation of organicmatter other than the sludge in the sewage treatment, since a largeamount of treatment water does not exist, hydrogen sulfide is removed bya method of dry adsorption with solid sorbents such as iron oxide andcarbon dioxide is removed by membrane separation or pressure swingadsorption (PSA).

[0005] Further, in the pretreatment of the digestion gas, an attempt hasalso been made to perform only desulfurization without the separation ofcarbon dioxide, i.e., without the enrichment of methane.

[0006] The method of wet absorption with a large amount of treatmentwater has an advantage that no chemical is necessary. However, thecarbon dioxide absorption capacity of the treatment water is so smallthat the size of the absorption apparatus has to be very large. Further,the dissolved oxygen and dissolved nitrogen in the treatment water aretransferred to the gas after the treatment, and therefore oxygen andnitrogen contained in the gas after the absorption amount to severalpercents, thus limit the level of the enrichment of methane, andadversely affect the subsequent hydrogen production step and the fuelcell power generation step.

[0007] On the other hand, in the PSA or the membrane separation, powerfor increasing or decreasing the pressure of gas is necessary, and, inaddition, the recovery rate of methane is low, disadvantageously leadingto a lowered energy efficiency of the system. As for the dry adsorptionof hydrogen sulfide, although the apparatus can be simplified, a highadsorption load of hydrogen sulfide causes an increase in running cost.

[0008] Further, in the case where the pretreatment does not involve theseparation of carbon dioxide, the power necessary for blowing andpressurizing the digestion gas to the subsequent step of reforming isincreased. In addition, the concentration of hydrogen in the reformedgas or hydrogen-containing gas is considerably low, and this poses aproblem that the energy efficiencies of the reforming step and the fuelcell power generation step are disadvantageously lowered.

[0009] Further, the concentration of methane in digestion gas variesfrom fermentation tank to fermentation tank used for the production ofmethane. Even in the same fermentation tank, the methane concentrationvaries with the season or a fluctuation in fermentation conditions orthe like, and the difference between the minimum concentration and themaximum concentration may reach 10%. A fluctuation of the methaneconcentration, i.e., the heating value of the gas supplied to thereforming step not only unstabilizes the operation of the hydrogenproduction step and thus the operation of the fuel cell power generationstep, but also significantly impairs the energy efficiency of the totalsystem. Therefore, it is important to keep the concentration of methanein the feed gas at a constant value as much as possible. In theconventional technology described above, however, it has been difficultor impossible to cope with this requirement.

DISCLOSURE OF INVENTION

[0010] The present invention has been made in view of the problems ofthe prior art, and it is an object of the present invention to provide ahighly efficient fuel cell power generation system having reducedenvironment load wherein a hydrogen-containing gas suitable for use in afuel cell, especially in a proton exchange membrane fuel cell isproduced from a digestion gas yielded in methane fermentation of organicmatter, and is then supplied to the fuel cell to generate electricity.

[0011] In order to achieve the above object, according to one aspect ofthe present invention, there is provided a fuel cell power generationmethod utilizing methane fermentation of organic matter, the methodcomprising: a methane fermentation step for subjecting organic matter tomethane fermentation to yield an anaerobic digestion gas; a pretreatmentstep for pretreating the digestion gas yielded in the methanefermentation step; a hydrogen production step for producing ahydrogen-containing gas from the pretreated gas in the pretreatmentstep; and a fuel cell power generation step for supplying thehydrogen-containing gas produced in the hydrogen production step to afuel cell to generate electricity; wherein the pretreatment stepcomprises an alkaline absorption step for absorbing carbon dioxide orcarbon dioxide and hydrogen sulfide contained in the digestion gas withan alkaline absorbent solution to separate carbon dioxide or carbondioxide and hydrogen sulfide from the digestion gas.

[0012] More specifically, in the alkaline absorption step, the digestiongas is brought into counterflow contact with the alkaline absorbentsolution to transfer carbon dioxide or carbon dioxide and hydrogensulfide into the alkaline absorbent solution to enrich the concentrationof methane; and the fuel cell power generation method furthercomprising: a heat exchange step for heating the alkaline absorbentsolution which has absorbed carbon dioxide or carbon dioxide andhydrogen sulfide therein in the alkaline absorption step, by waste heatgenerated in the hydrogen production step and/or the fuel cell powergeneration step; and a regeneration step for regenerating the alkalineabsorbent solution by bringing the alkaline absorbent solution heated inthe heat exchange step into counterflow contact with a cathode vent gasdischarged from the fuel cell power generation step or a combustionexhaust gas discharged from the hydrogen production step to strip carbondioxide or carbon dioxide and hydrogen sulfide from the alkalineabsorbent solution. The gas after the absorption operation may bebrought into counterflow contact with water to wash away absorbentsolution droplets carried over into the gas. Further, the fuel cellpower generation method according to the present invention may furthercomprise a desulfurization step wherein the digestion gas yielded in themethane fermentation step is brought into counterflow contact with adesulfurizing solution containing an alkali chemical and a water-solubleoxidizing agent to absorb and oxidatively decompose hydrogen sulfide,and is then supplied to the alkaline absorption step.

[0013] According to the present invention, the concentration of methanein the gas after the absorption operation can be controlled at aconstant value by varying the flow rate and/or the temperature of theabsorbent solution in the alkaline absorption step.

[0014] In the present invention, the hydrogen production step comprisesa reforming step and a carbon monoxide shift reaction step. A carbonmonoxide selective oxidation step may be additionally provided after thereforming step and the carbon monoxide shift reaction step. A protonexchange membrane fuel cell or a phosphoric acid fuel cell is suitableas the fuel cell used in the fuel cell power generation step.

[0015] The digestion gas yielded in the methane fermentation of organicmatter varies with the type of the organic matter and methanefermentation conditions. In general, however, the digestion gascomprises 60 to 70% of methane as a main component, 30 to 40% of carbondioxide, 0 to 2% of hydrogen, and 0 to 2% of nitrogen, and furthercontains several tens to several hundreds of ppm of hydrogen sulfide andhydrogen chloride as minor components.

[0016] However, as a raw gas supplied to the hydrogen production step,low hydrogen sulfide concentration, low oxygen concentration, and lownitrogen concentration, and methane concentration which is constant andis high as much as possible are required.

[0017] As described above, according to the present invention, the useof an alkaline absorbent solution having a high absorption capacity suchas an absorbent solution of amine in the absorption of carbon dioxide orcarbon dioxide and hydrogen sulfide can provide a compact absorptionapparatus. The use of the alkaline absorbent solution can realize amethane recovery of nearly 100%, and can avoid the mixing of nitrogengas, oxygen gas or the like in the methane gas. Therefore, theconcentration of methane in the gas after the absorption can be steadilyraised to 95% or more.

[0018] As a gas for the regenerating of the alkaline absorbent solution,the use of a cathode vent gas discharged from the fuel cell powergeneration step or a combustion exhaust gas discharged from the hydrogenproduction step makes it possible to utilize waste heat possessed by thecombustion exhaust gas from the hydrogen production step and/or a partof waste heat with a poor value from the fuel cell power generationstep, in heating the absorbent solution for its regeneration. Therefore,the alkaline absorbent solution can be regenerated by properly settingabsorption conditions and regeneration conditions.

[0019] An anode vent gas discharged from the fuel cell power generationstep is used as a fuel for maintaining the reforming temperature andsupplying the reforming heat in the reforming step within the hydrogenproduction step. Here, there are two cases, one case where air is usedas a combustion-supporting gas, and another case where a cathode ventgas discharged from the fuel cell power generation step is used as thecombustion-supporting gas. When the cathode vent gas is used as thecombustion-supporting gas, since the combustion exhaust gas dischargedfrom the hydrogen production step has a carbon dioxide concentration ofnot more than 6%, this combustion exhaust gas can be utilized as aregeneration gas by properly setting the conditions for the regenerationof the alkaline absorbent solution in a regenerator within theabsorption step. Specifically, according to the present invention, whenthe cathode vent gas is used as the combustion-supporting gas, thecombustion exhaust gas is used as the regeneration gas, while, when airis used as the combustion-supporting gas, the combustion exhaust gas isutilized as a heat source for the alkaline absorbent solution at thetime of the regeneration without the use of the combustion exhaust gasas the regeneration gas. In this case, the cathode vent gas is utilizedas the regeneration gas. Furthermore, air can be additionally used asthe regeneration gas.

[0020] Due to the mass balance of the system according to the presentinvention, the concentration of carbon dioxide in the regenerationoff-gas discharged from the regenerator is not more than 10% when thecombustion exhaust gas is used as the regeneration gas, and is not morethan 5% when the cathode vent gas is used as the regeneration gas.Further, due to the heat balance of the system according to the presentinvention, heat necessary for the regeneration of the alkaline absorbentsolution is not more than 50% of the stack waste heat discharged fromthe fuel cell power generation step.

[0021] The absorption conditions and regeneration conditions in thealkaline absorption step are set such that the temperature of thealkaline absorbent solution prior to regenerating in the regenerationstep is lower than the temperature of the outlet of stack cooling waterfrom the fuel cell power generation step by 10° C. or less, preferably2° C. or less, while the temperature of the alkaline absorbent solutionprior to absorbing in the alkaline absorption step is lower than thetemperature of the alkaline absorption solution prior to regenerating inthe range of 10° C. to 35° C., and is higher than the temperature of amethane fermentation liquid in the methane fermentation step bypreferably 2° C. or more. The temperature of the outlet of stack coolingwater varies with the type of the fuel cell used. This temperature ofthe outlet of stack cooling water, however, is generally in the range of60 to 80° C. While in the case of a proton exchange membrane fuel cellusing a high temperature-type

having a heat-resistant temperature of 100 to

0° C., the temperature of the outlet of stack cooling water is in therange of 100 to 120° C.

[0022] Thus, according to the present invention, the temperature of thealkaline absorbent solution prior to regenerating and the temperature ofthe alkaline absorbent solution prior to absorbing are properly set, andthen the alkaline absorbent solution from the regeneration step isbrought to the methane fermentation step to perform heat exchange with amethane fermentation liquid for thereby bringing the temperature of thealkaline absorbent solution to a predetermined temperature prior toabsorbing, followed by supplying the alkaline absorbent solution to theabsorption step. By virtue of this constitution, the methanefermentation liquid can be utilized as a coolant for the alkalineabsorbent solution, and, in addition, the waste heat generated in thefuel cell power generation step can be utilized, in a cascade form, as aheat source for the regeneration of the alkaline absorbent solution andas a heat source for heating the methane fermentation liquid andretaining the temperature of the methane fermentation liquid. Themethane fermentation liquid is large in quantity and stable intemperature, and hence is very suitable as a coolant for adjusting thetemperature of the alkaline absorbent solution from the temperatureafter the regeneration to a predetermined temperature prior toabsorbing. Therefore, the provision of a cooling tower or an air cooleris unnecessary in the pretreatment step according to the presentinvention.

[0023] Thus, the alkaline absorption step according to the presentinvention can be realized in terms of both mass balance and heatbalance. Therefore, the present invention can enhance the energyefficiency of the system.

[0024] Further, the use of the alkaline absorption step or thedesulfurization step according to the present invention for removinghydrogen sulfide can reduce the running cost even in the case of highhydrogen sulfide concentration.

[0025] In the present invention, each of the following methods may beadopted: a method wherein a desulfurization step is provided forhydrogen sulfide and the digestion gas is brought into couterflowcontact with a hydrogen sulfide absorbent solution containing an alkalichemical and a water-soluble oxidizing agent in a desulfurizer to absorband decompose hydrogen sulfide; and a method wherein the desulfurizationstep is not provided and hydrogen sulfide, together with carbon dioxide,is absorbed and separated in an alkaline absorption step. As a matter ofcourse, when the hydrogen sulfide concentration is relatively low,method of dry adsorption using iron oxide or the like may be used.

[0026] Conventionally, in desulfurizing the digestion gas, it is acommon practice to use a combination of a primary dry adsorptiondesulfurization after the fermentation step and a deep dry adsorptiondesulfurization before the hydrogen production step. Therefore, thisconventional method is disadvantageous in that, particularly when thehydrogen sulfide load is high, the adsorbent has to be frequentlyreplaced and this is troublesome and incurs high running cost. Bycontrast, according to the present invention, it is possible to perform,in a single step (i.e., the alkaline absorption step), thedesulfurization of the digestion gas, the separation of carbon dioxidefrom the digestion gas, i.e., methane enrichment, and the stabilizationof the concentration of methane in the pretreated gas. This can realizea simplified process with a low running cost.

[0027] When the method in which hydrogen sulfide is absorbed and removedin the alkaline absorption step is employed, the absorbed hydrogensulfide is accumulated in the absorbent solution in an early stage ofthe operation. However, after the concentration of hydrogen sulfide inthe absorbent solution reaches a certain value, the hydrogen sulfide,together with carbon dioxide, is stripped in the alkaline absorbentsolution regeneration step and is transferred to the regenerationoff-gas. According to the present invention, particularly when theconcentration of hydrogen sulfide in the digestion gas is high, i.e.,when the load of hydrogen sulfide is high, a biodesulfurization stepcharacterized by low running cost can be provided as the desulfurizationmeans to biodecompose hydrogen sulfide contained in the regenerationoff-gas.

[0028] According to the biodecomposition of hydrogen sulfide, hydrogensulfide is oxidized by microorganisms into sulfur or sulfuric acid, andhence the supply of oxygen is necessary. Conventionally, thebiodesulfurization could not be adopted as the method for thedesulfurization of digestion gas, because the introduction of air oroxygen into the digestion gas adversely affects the subsequent hydrogenproduction step. By contrast, according to the present invention, asdescribed above, hydrogen sulfide contained in the digestion gas is oncetransferred from the digestion gas to the regeneration off-gas, andhence the adoption of the biodesulfurization becomes possible. Inaddition, according to the present invention, since an oxygen-containingcombustion exhaust gas discharged from the hydrogen production step oran oxygen-containing cathode vent gas discharged from the fuel cellpower generation step is used as the regeneration gas for the alkalineabsorbent solution, the use of the biodesulfurization is highlysuitable.

[0029] On the other hand, in the regeneration of the alkaline absorbentsolution, the absorbent solution comes into contact with theregeneration gas, and, in this case, there is a fear that an amine-basedabsorbent is reacted with oxygen contained in the regeneration gas andthus is degraded by the oxidation. However, because the reactivity ofhydrogen sulfide with oxygen is higher than that of the amine-basedabsorbent with oxygen, hydrogen sulfide contained in the absorbentsolution can act as an antioxidant of the amine-based absorbent. Thus,according to the present invention, proper adjustment of the load ofhydrogen sulfide in the alkaline absorption step can ensure completedesulfurization of the gas after the absorption and, at the same time,can suppress the degradation of the amine-based absorbent by oxidation.

[0030] When carbon dioxide and/or hydrogen sulfide are absorbed andseparated by an absorption method, as the regeneration gas for theabsorbent solution, it is a common practice to use steam, or otherwiseair when steam is not available. While in the present invention, thecombustion exhaust gas from the hydrogen production step or the cathodevent gas from the fuel cell power generation step is used as theregeneration gas, and this is based on the following consideration.Specifically, although the use of steam facilitates the regeneration ofthe alkaline absorbent solution, the provision of a high temperatureheat source of 100° C. or above is necessary for generating steam.Further, air is advantageous as the regeneration gas in that carbondioxide is not substantially contained, but the air is disadvantageousin that the temperature and the absolute humidity are so low that theabsorbent solution being regenerated, temperature of which should beessentially kept high, is cooled due to steam evaporation, and that thehigh partial pressure of oxygen potentially enhances the oxidicdegradation of the amine-based absorbent. By contrast, according to thepresent invention, the combustion exhaust gas from the hydrogenproduction step and the cathode vent gas from the fuel cell powergeneration step, which are used under properly set conditions ofabsorption and regeneration, have higher temperature and absolutehumidity, that is, higher enthalpy, and lower partial pressure of oxygenas compared with air, and hence are very advantageous as theregeneration gas, so using of these gases eliminates the need of hightemperature heat source.

[0031] Thus, the present invention can enhance the energy efficiency ofthe fuel cell power generation system and can improve the profitability.

BRIEF DESCRIPTION OF DRAWINGS

[0032]FIG. 1 is a block diagram of the fuel cell power generation systemaccording to the first embodiment of the present invention;

[0033]FIG. 2 is a block diagram of the fuel cell power generation systemaccording to the second embodiment of the present invention;

[0034]FIG. 3 is a block diagram of the fuel cell power generation systemaccording to the third embodiment of the present invention;

[0035]FIG. 4 is a block diagram showing a fuel cell power generationsystem according to an example of the first embodiment shown in FIG. 1;

[0036]FIG. 5 is a block diagram showing a fuel cell power generationsystem according to an example of the second embodiment shown in FIG. 2;

[0037]FIG. 6 is a block diagram showing a fuel cell power generationsystem according to an example of the third embodiment shown in FIG. 3;

[0038]FIG. 7 is a block diagram showing a fuel cell power generationsystem according to the second example of the second embodiment shown inFIG. 2; and

[0039]FIG. 8 is a block diagram of the fuel cell power generation systemaccording to the fourth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] A fuel cell power generation system according to the firstembodiment of the present invention will be described in detail withreference to the accompanying drawings.

[0041]FIG. 1 is a schematic block diagram showing the fuel cell powergeneration system according to the first embodiment.

[0042] As shown in FIG. 1, in this embodiment, organic matter a isfermented in a methane fermentation step A. An anaerobic digestion gas bproduced in the methane fermentation step A is pretreated in apretreatment step B, and the pretreated gas is processed in a hydrogenproduction step C to produce a hydrogen-containing gas c which is thensupplied to a fuel cell power generation step D to generate electricity.

[0043] The hydrogen production step C comprises a reforming step C1 forcatalytically reforming methane, contained in the gas pretreated in thepretreatment step B, with steam into hydrogen and carbon monoxide; ashift reaction step C2 for catalytically converting carbon monoxide,contained in the gas after the reforming, with steam into hydrogen gasand carbon dioxide; and a selective oxidation step C3 for catalyticallyreacting carbon monoxide, remaining in the gas after the shifting, withan introduced oxygen-containing gas to selectively oxidize and removethe residual carbon monoxide.

[0044] Here, a combustion exhaust gas d discharged from the reformingstep C1 in the hydrogen production step C is utilized as a regenerationgas in an alkaline absorption step B1. A part of stack waste heat g fromthe fuel cell power generation step D is utilized for heating thealkaline absorbent solution for its regeneration, while the remainingpart of the stack waste heat g is utilized for heating the methanefermentation liquid in the methane fermentation step A.

[0045] These steps will be explained in more detail.

[0046] A) Methane Fermentation Step

[0047] According to the present invention, organic matter, especiallyorganic wastes, such as waste water discharged from food production,waste water from livestock farms, or excess sludge yielded in abiological treatment of sewage or the like, is subjected to methanefermentation to yield a digestion gas which is then used to produce ahydrogen gas or a hydrogen-containing gas. The hydrogen gas or thehydrogen-containing gas is then supplied to a fuel cell to generateelectricity, while waste heat generated in the fuel cell powergeneration step is utilized as a heat source for the methanefermentation step.

[0048] In the methane fermentation step A, digestion by microorganismsunder anaerobic conditions for a residence time of 20 to 30 daysdecomposes about 50% of the organic matter according to the followingreaction formula to produce a methane gas and carbon dioxide.

Organic matter→fatty acid→CH₄+CO₂  (1)

[0049] There is no particular limitation on conditions for the methanefermentation step A. However, medium-temperature fermentation at atemperature of 30 to 35° C. is suitable from the viewpoints of theresidence time and the efficiency. In this case, stack waste heat g(described below) generated in the fuel cell power generation step D andheat exchanged in the cooling of the alkaline absorbent solution afterthe regeneration in the alkaline absorption step B1 are used as the heatsource for heating the fermentation liquid and maintaining thetemperature of the fermentation liquid.

[0050] B) Pretreatment Step-Alkaline Absorption Step

[0051] According to the present invention, an alkaline absorption stepB1 is provided as the pretreatment step B. In this step, the digestiongas is led to an absorber, where the digestion gas is brought intocounterflow contact with an alkaline absorbent solution to absorb carbondioxide or carbon dioxide and hydrogen sulfide (hereinafter referred toas “carbon dioxide and the like”), thereby separating carbon dioxide andthe like from the digestion gas. The alkaline absorbent solution, whichhas been discharged from the absorber, is heated by waste heat d, ggenerated in the hydrogen production step C and/or the fuel cell powergeneration step D, and is then transferred to the regenerator where thealkaline absorbent solution is regenerated with a combustion exhaust gasd discharged from the hydrogen production step C or the cathode vent gasf discharged from the fuel cell power generation step D. The regeneratedalkaline absorbent solution is returned to the absorber.

[0052] The absorber and the regenerator and a rinsing tower and adesulfurizer (described below) are packed with packing. Any type ofpacking may be used as long as the packing has satisfactory corrosionresistance and heat resistance and, at the same time, has high contactefficiency. An absorbent solution of potassium carbonate or an absorbentsolution of an alkanolamine is suitable as the absorbent solution usedin the absorber. According to the present invention, the absorbentsolution of an alkanolamine is more preferred, because this absorbentsolution has high absorption capacity and, in addition, can performabsorption and regeneration at a temperature under 80° C. which can beprovided by utilizing the waste heat g with a poor value generated inthe fuel cell power generation step D and/or the waste heat of thecombustion exhaust gas d discharged from the hydrogen production step C.Specific examples of absorbents applicable herein includemonoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine(MDEA) and the like. A reaction involved in the absorption of carbondioxide into the absorbent solution of an alkanolamine is represented bythe following formula:

R—NH₂+H₂O+CO₂→R—NH₃HCO₃  (2)

[0053] The regeneration reaction is a reverse reaction of the reaction(2).

[0054] The gas which has passed through the alkaline absorption step B1is transferred to the hydrogen production step C as the next step. Inthis case, when the content of sulfur in the gas has not beensatisfactorily lowered, a dry adsorption desulfurizer may be providedbetween the alkaline absorption step B1 and the hydrogen production stepC.

[0055] C) Hydrogen Production Step

[0056] 1) Reforming Step

[0057] According to the present embodiment, a reforming step C1 isprovided in the hydrogen production step C to perform the followingsteam reforming reaction in a reforming reactor packed with a reformingcatalyst.

CH₄+H₂)→CO+3H₂  (3)

[0058] Steam generated in a steam boiler using, as a heat source, thesensible heat of the reformed gas is added as the steam necessary forthe reaction. The amount of the steam added is preferably in the rangeof 2.5 to 3.5 in terms of the molar ratio of steam to methane, i.e., S/Cratio. This reforming reaction is an endothermic reaction. Therefore, ahigher reaction temperature lowers the equilibrium concentration ofmethane and enhances the reaction rate, while leads to a lower thermalefficiency. For this reason, the reaction temperature is preferably inthe range of 650 to 800° C. The supply of reaction heat and theretention of the reaction temperature are carried out by usingcombustion heat of an anode vent gas e, discharged from the fuel cellpower generation step D. The type and form of the catalyst are notparticularly limited as long as the reforming reaction can beaccelerated. Catalysts suitable for use in the above temperature rangeinclude nickel-based, ruthenium-based, platinum-based,nickel-ruthenium-based, and ruthenium-platinum-based steam reformingcatalysts and composite ones of these catalysts.

[0059] 2) Shift Reaction Step

[0060] Further, a shift reaction step C2 is provided in the hydrogenproduction step C to perform the following shift reaction in a shiftreactor packed with a shift reaction catalyst.

CO+H₂O→CO₂+H₂  (4)

[0061] Steam contained in the gas after reforming is utilized as steamnecessary for the reaction. The shift reaction is an exothermicreaction. Therefore, a lower reaction temperature lowers the equilibriumconcentration of carbon monoxide, while brings the reaction rate down.For this reason, the reaction temperature is preferably in the range of200 to 250° C. The type and form of the catalyst are not particularlylimited as long as the shift reaction can be accelerated. Catalystssuitable for use in the above temperature range include copper-zinc-baseshift reaction catalysts.

[0062] 3) Selective Oxidation Step

[0063] When a proton exchange membrane fuel cell is employed in the fuelcell power generation step D, a selective oxidation step C3 is providedin the hydrogen production step C to further lower the concentration ofcarbon monoxide in the gas after the shift reaction to not more than 100ppm, preferably not more than 10 ppm. That is, the following selectiveoxidation reaction is carried out by supplying an oxygen gas to aselective oxidation reactor packed with a selective oxidation catalystwhile leading the above gas to the selective oxidation reactor.

CO+½O₂→CO₂  (5)

[0064] The larger the amount of oxygen supplied, the lower theconcentration of the residual carbon monoxide. The amount of oxygen,however, is preferably around 2 equivalents to carbon monoxide. Thereaction temperature is preferably in the range of 100 to 150° C. Anycatalyst may be used without particular limitation as long as thecatalyst has high selectivity for oxidation of carbon monoxide and canrealize high reaction rate. For example, a platinum-based catalyst or agold catalyst with gold supported on alumina is suitable.

[0065] D) Fuel Cell Power Generation Step

[0066] According to this embodiment, the hydrogen-containing gas cproduced in the hydrogen production step C has a relatively lowtemperature, a high hydrogen concentration, and a low carbon monoxidecontent. Therefore, the fuel cell employed is preferably a phosphoricacid fuel cell or especially a proton exchange membrane fuel cell,operatable at a relatively low temperature. Electrochemical reactionsinvolved in the phosphoric acid or proton exchange membrane fuel cellare as follows:

Anode reaction: H₂→2H⁺+2e ⁻  (6)

Cathode reaction: ½O₂+2H⁺+2e ⁻→H₂O  (7)

[0067] More specifically, the hydrogen-containing gas c is supplied tothe anode compartment in a fuel cell stack while an oxygen-containinggas h is supplied to the cathode compartment, whereby electricity isgenerated by the reactions in the cell. The operating temperature of thephosphoric acid fuel cell and the operating temperature of the protonexchange membrane fuel cell are around 200° C. and around 80° C.,respectively. Since, however, these reactions are exothermic reactions,the stacks should be cooled to maintain the above operatingtemperatures. According to the present invention, outlet of stackcooling water (low-temperature warm water in the case of the phosphoricacid fuel cell) g is used as a heat source for the regeneration of thealkaline absorbent solution and as a heat source for the methanefermentation liquid. Thus, the energy efficiency of the whole system canbe enhanced.

[0068] Further, in the fuel cell power generation, in order to ensurethe power generation efficiency and the durability of the stack, it is acommon practice to leave about 30% of the hydrogen gas supplied to theanode compartment in the stack without consuming 100% of the suppliedhydrogen gas and to discharge it as an anode vent gas e from the stack.According to this embodiment, as described above, the anode vent gas eis utilized as a fuel in the reforming step C1, while the dischargedcombustion exhaust gas and the cathode vent gas f are utilized for theregeneration of the alkaline absorbent solution in the regenerationstep.

[0069] Next, an example of the first embodiment will be described withreference to FIG. 4. FIG. 4 is a diagram showing the fundamentalconstitution of the fuel cell power generation system according to thisexample.

[0070] At the outset, organic matter a is fermented in a fermentationtank 11 for the methane fermentation step A to yield a digestion gas b.The digestion gas b is passed through a digestion gas holder 14 and isled to the bottom of an absorber 21 for performing an alkalineabsorption step B1. In this absorber 21, the digestion gas b is broughtinto counterflow contact with the alkaline absorbent solution 22 toabsorb and remove carbon dioxide and hydrogen sulfide. In this case,carbon dioxide is removed by absorption so that the concentration of theresidual carbon dioxide in the gas 23 after the absorption is not morethan 10%, preferably not more than 5%, while hydrogen sulfide is removedby absorption so that the concentration of the residual hydrogen sulfidein the gas 23 after the absorption is not more than 10 ppm, preferablynot more than 1 ppm, more preferably not more than 0.1 ppm.

[0071] The gas 23 after the absorption, which has been discharged fromthe head of the absorber 21, enters a rinsing tower 31 installed at theupper part of the absorber 21, where the gas 23 is brought intocounterflow contact with rinsing water 32 to wash away absorbentsolution droplets carried over from the absorber 21. The gas 33 afterthe rinsing operation, which has been discharged from the rinsing tower31, is transferred by a blower 34 to the reforming step C1 in thehydrogen production step C as the next step. According to the presentinvention, in the alkaline absorption step B1, carbon dioxide containedin the digestion gas b can be separated and removed by 90% or more.Therefore, as compared with the case where the alkaline absorption stepis not provided, the power of blower 34 for blowing and boosting can bereduced by about 40%.

[0072] As a result of the rinsing operation, the concentration of thealkaline absorbent in the rinsing water 35 is gradually increased.Therefore, in this example, a part of the rinsing water 35 is alwayswithdrawn through a valve 36, and pure make-up water 38 in an amountcorresponding to the withdrawn rinsing water 37 is added to the rinsingwater. The withdrawn rising water 37 is utilized as make-up water forthe alkaline absorbent solution 22. Further, condensate recovered bycooling and condensate separating of the regeneration off-gas 47 alsomay be used as the make-up water 38.

[0073] Here, the alkaline absorbent solution 24 with carbon dioxide andhydrogen sulfide being absorbed thereinto (hereinafter referred to asthe “alkaline absorbent solution after the absorption operation”) istransferred from the bottom of the absorber 21 to a heat exchanger 26,for a heat exchange step, by a liquid delivery pump 25. In this heatexchanger 26, the heat exchange step is carried out. More specifically,the heat exchange between the alkaline absorbent solution 24 after theabsorption operation and the outlet of stack cooling water 39 from thefuel cell power generation step D is carried out, whereby the alkalineabsorbent solution 24 after the absorption operation is heated to atemperature lower than the temperature of the outlet of stack coolingwater 39 by 10° C. or less, preferably about 2° C. or less.

[0074] The alkaline absorbent solution 27 thus heated is led to theinlet at the top of a regenerator 41 for a regeneration step. In theregenerator 41, the heated alkaline absorbent solution 27 is broughtinto counterflow contact with the combustion exhaust gas d dischargedfrom the reforming step C1. As a result, carbon dioxide and hydrogensulfide absorbed into the alkaline absorbent solution are stripped fromthe alkaline absorbent solution, and hence the alkaline absorbentsolution is regenerated.

[0075] The alkaline absorbent solution 42 after the regenerationoperation (hereinafter referred to as the “regenerated alkalineabsorbent solution”) is transferred from the bottom of the regenerator41 to the heat exchanger 13 in the methane fermentation tank 11 by aliquid delivery pump 43. This heat exchanger 13 cools the regeneratedalkaline absorbent solution 42 to a temperature which is lower than thetemperature of the alkaline absorbent solution 27 (before theregeneration) at the inlet at the top of the regenerator 41 by about 10to 35° C., and is higher than the temperature of the methanefermentation liquid contained in the fermentation tank 11 by 2° C. ormore. The cooled alkaline absorbent solution 22 is again led to the topof the absorber 21. This permits a considerable amount of the heatsupplied for the regeneration to be reused for heating the methanefermentation liquid. It should be noted that, instead of the heatexchanger 13, a water cooler or an air cooler also may be used to coolthe regenerated alkaline absorbent solution 42.

[0076] The repeated use of the alkaline absorbent solution results in agradual degradation of some active component. Therefore, in thisexample, a part of the regenerated alkaline absorbent solution 42 isalways withdrawn through a valve 44, and a fresh absorbent solution inan amount corresponding to the withdrawn solution 45 is introducedthrough a chemical feed unit 46 into the system. Here, a part ofhydrogen sulfide accumulated in the alkaline absorbent solution 27 isoxidized with oxygen contained in the combustion exhaust gas d for theregeneration, and there is a fear that sulfuric acid formed by thisoxidation lowers the pH value of the absorbent solution. In this case,blending potassium hydroxide or sodium hydroxide into the freshabsorbent solution to be supplied allows the pH value of the absorbentsolution being kept constant.

[0077] The gas 33 after the rinsing operation is subjected to steamreforming in the reforming step C1, and is then passed through thecarbon monoxide shift reaction step C2 and the carbon monoxide selectiveoxidation step C3 to produce a hydrogen-containing gas. Next, thehydrogen-containing gas, and air h as an oxidizing agent are supplied tothe fuel cell power generation step D, where fuel cell power generationis carried out. In this case, when the fuel cell employed in the fuelcell power generation step D is a phosphoric acid fuel cell, the carbonmonoxide selective oxidation step C3 is unnecessary.

[0078] In this example, both the anode vent gas e and the cathode ventgas f discharged from the fuel cell power generation step D are suppliedto a burner in the reforming step C1, and, as described above, thecombustion exhaust gas d discharged from the reforming step C1 isutilized as a regeneration gas in the regenerator 41 of the regenerationstep.

[0079] Further, as described above, after the heat exchange between theoutlet of stack cooling water 39 from the stack in the fuel cell powergeneration step D, and the alkaline absorbent solution 24 after theabsorption operation is carried out with the heat exchanger 26, and theheat exchange between the outlet of stack cooling water 39 and themethane fermentation liquid is further carried out with the heatexchanger 12 in the methane fermentation tank 11, the outlet of stackcooling water 39 is thus brought to a predetermined temperature, andthen is returned as inlet of cooling water 40 to the stack.

[0080] Next, the second embodiment of the fuel cell power generationsystem according to the present invention will be described in detailwith reference to the accompanying drawings.

[0081]FIG. 2 is a schematic block diagram showing the fuel cell powergeneration system according to this embodiment. In FIG. 2, steps andelements identical to or corresponding to those in FIG. 1 have the samereference characters as used in FIG. 1. In the second embodiment,portions not specifically referred to herein are the same as those inthe first embodiment.

[0082] According to this embodiment, as shown in FIG. 2, the combustionexhaust gas d discharged from the reforming step C1 in the hydrogenproduction step C is utilized as a heat source for regeneration in thealkaline absorption step B1, while the cathode vent gas f dischargedfrom the fuel cell power generation step D is utilized as a regenerationgas in the alkaline absorption step B1. Further, a part of the stackwaste heat g from the fuel cell power generation step D is utilized forheating the alkaline absorbent solution for its regeneration, while theremainder of the stack waste heat g is utilized for heating the methanefermentation liquid in the methane fermentation step A.

[0083] Next, the first example of the second embodiment will bedescribed with reference to FIG. 5. FIG. 5 shows the fundamentalconstitution of the fuel cell power generation system according to thisexample.

[0084] In this example, since air i is used as a combustion-supportinggas in the reforming step C1 with the hydrogen production step C, thecombustion exhaust gas d discharged from the reforming step C1 is led toan additionally installed heat exchanger 28. A heat exchange step iscarried out also with this heat exchanger 28. More specifically, afterthe heat exchange with the alkaline absorbent solution 24, thecombustion exhaust gas 29 after the heat exchange is exhausted. Here,the cathode vent gas f discharged from the fuel cell power generationstep D is led as a regeneration gas to the regenerator 41. Theconstitution except points referred above is the same as theconstitution of the example of the first embodiment, and thus theexplanation thereof will be omitted.

[0085] Next, the second example of the second embodiment will bedescribed with reference to FIG. 7. This example is the same as thefirst example, except that means for controlling the concentration ofmethane in the gas 33 after the absorption operation to a certain valueis incorporated.

[0086] As shown in FIG. 7, in this example, there are provided methaneconcentration detection means 60, level detection means 61 for the levelof the solution stored in the tank of the regenerator 41, a controller62, and inverters 63, 64. For example, when the concentration of methanein the digestion gas b gets down due to some causes, the concentrationof methane in the gas 33 after the absorption operation also tends toget down if no measure is taken. In this example, this unfavorablephenomenon can be avoided by the following procedure. Specifically, thetendency of decreasing in the methane concentration is detected by themethane concentration detection means 60, and a detection signal is sentto the controller 62. The controller 62 compares the detection signalwith the set value, and sends a control signal to the inverter 63 so asto increase the frequency. Then, the inverter 63 raises the rotationalspeed of the pump 43 to increase the flow rate of the alkaline absorbentsolution 22 supplied to the absorber 21. This in turn increases theabsorption capacity of the absorber 21, and thus results in maintainingthe concentration of methane in the gas 33 to a certain value. However,this brings a decreasing tendency to the level of the solution stored inthe tank of the regenerator 41. Here, the detection signal output fromthe level detection means 61 is sent to the controller 62 which thensends a control signal to the inverter 64 to raise the rotational speedof the pump 25. Thus, the level of the solution stored in the tank iskept unchanged. According to this example, even in the situation whenthe concentration of methane in the digestion gas b gets high due tosome causes, the concentration of methane in the gas 33 can also be keptconstant, although a detailed explanation thereof is omitted.

[0087] Any detection means may be used as the methane concentrationdetection means as long as the methane concentration can be detected.However, a continuous infrared absorption-type methane concentrationmeter is preferable. Further, carbon dioxide detection means may be usedinstead of the methane concentration detection means. Further, insteadof the methane concentration detection means or the carbon dioxidedetection means, hydrogen concentration detection means (not shown inFIG. 7) for .detecting the concentration of hydrogen in thehydrogen-containing gas c obtained from the hydrogen production step Cmay also be used for controlling the hydrogen concentration to a certainvalue, so as to secure a steady and efficient operation in the fuel cellpower generation step D.

[0088] The capacity of the alkaline absorbent solution 22 to absorbcarbon dioxide increases with lowering its temperature. Therefore,according to the present invention, in addition to the flow rate of thealkaline absorbent solution in the absorber, the temperature of theabsorbent solution can also be used as an operational leverage forcontrolling the concentration of methane in the gas 33 after theabsorption operation or the concentration of hydrogen in thehydrogen-containing gas c after the hydrogen production step C, althoughthis is not shown in the drawing. Of course, the combination of the flowrate and the temperature of the absorbent solution may also be used.

[0089] Next, the third embodiment of the fuel cell power generationsystem according to the present invention will be described in detailwith reference to the accompanying drawings.

[0090]FIG. 3 is a schematic block diagram showing the fuel cell powergeneration system according to this embodiment. In FIG. 3, steps andelements identical to or corresponding to those in FIG. 1 have the samereference characters as used in FIG. 1. In the third embodiment,portions not specifically referred to herein are the same as those inthe first embodiment.

[0091] According to this embodiment, as shown in FIG. 3, the combustionexhaust gas d discharged from the reforming step C1 within the hydrogenproduction step C is utilized as a regeneration gas in the alkalineabsorption step B1. In this system, of course, the combustion exhaustgas d discharged from the reforming step C1 may be utilized as a heatsource for the regeneration in the alkaline absorption step B1, whilethe cathode vent gas f discharged from the fuel cell power generationstep D may be utilized as a regeneration gas in the alkaline absorptionstep B1. Further, a part of the stack waste heat g discharged from thefuel cell power generation step D is utilized for heating the alkalineabsorbent solution for its regeneration, while the remainder of thestack waste heat g is utilized for heating the methane fermentationliquid in the methane fermentation step A.

[0092] According to this embodiment, there is provided a desulfurizationstep E in which the digestion gas b yielded in the methane fermentationstep A is brought into counterflow contact with a desulfurizing solutioncontaining an alkali chemical and a water-soluble oxidizing agent toabsorb and oxidatively decompose hydrogen sulfide. For example, causticsoda and potassium hydroxide are suitable as the alkali chemical, andchlorine-based oxidizing agent, bromine-based oxidizing agent, andhydrogen peroxide are suitable as the water-soluble oxidizing agent.When caustic soda is used as the alkali chemical with sodium hypobromitebeing used as the water-soluble oxidizing agent, the followingabsorption reaction and oxidative decomposition reaction take place.

H₂S+NaOH→NaHS+H₂O  (8)

NaHS+3NaOBr→NaHSO₃+3NaBr  (9)

[0093] An example of the third embodiment will be described withreference to FIG. 6. FIG. 6 shows the fundamental constitution of thefuel cell power generation system according to this example.

[0094] In this example, the digestion gas b is introduced into adesulfurizer 51 for a desulfurization step E. In the desulfurizer 51,the digestion gas b is brought into counterflow contact with adesulfurizing solution 52. This operation permits hydrogen sulfide to beabsorbed and oxidatively decomposed so that the concentration ofhydrogen sulfide in the gas 53 after the desulfurization operation isnot more than 10 ppm, preferably not more than 1 ppm, more preferablynot more than 0.1 ppm. As a result of the desulfurization operation, thedesulfurizing solution 52 is gradually deteriorated. Therefore, in thisexample, a part of the desulfurizing solution 52 is always withdrawnthrough a valve 54, and a fresh desulfurizing solution in an amountcorresponding to the withdrawn solution 55 is introduced into the systemthrough a chemical feed unit 56.

[0095] The gas 53 desulfurized in the desulfurizer 51 is then sent tothe absorber 21 for the alkaline absorption step B1. Operation after thetreatment in the absorber 21 is the same as that in the first example ofthe first embodiment, and thus the explanation thereof will be omitted.

[0096] Next, the fourth embodiment of the fuel cell power generationsystem according to the present invention will be described in detailwith reference to the accompanying drawings.

[0097]FIG. 8 is a schematic block diagram showing the fuel cell powergeneration system according to this embodiment. In FIG. 8, steps andelements identical to or corresponding to those in FIG. 2 have the samereference characters as used in FIG. 2. In the fourth embodiment,portions not specifically referred to herein are the same as those inthe second embodiment.

[0098] According to this embodiment, as shown in FIG. 8, a hydrogensulfide biodegradation step F is provided. The regeneration off-gasdischarged from the alkaline absorption step B is led to the hydrogensulfide biodegradation step F to biodegrade, by microorganisms, hydrogensulfide contained in the gas into sulfur or sulfuric acid. The gas afterthe biodesulfurization operation is discharged as a final exhaust gas.

[0099] As described above, according to the present invention, thedesulfurization, the separation of carbon dioxide, and the stabilizationof methane concentration of the digestion gas yielded in the methanefermentation of organic matter can be simultaneously carried out in asingle step, i.e., in the alkaline absorption step. This can realize asimplified fuel cell power generation system using a digestion gas,coupled with a reduced running cost. Further, since methane in adigestion gas can be steadily enriched to a certain concentration up to95% or even higher, the hydrogen production step and the fuel cell powergeneration step can be stably operated. Further, the recovery rate ofmethane is approximately 100%, and the power consumption is also small.In addition, waste heat with a poor value generated in the hydrogenproduction step and the fuel cell power generation step can be utilizedin a cascade form. Therefore, the energy efficiency of the whole systemis high.

[0100] Although only a few certain preferred embodiments of the presentinvention have been shown and described in detail, it should beunderstood that various changes and modifications may be made thereinwithout departing from the scope of the appended claims.

INDUSTRIAL APPLICABILITY

[0101] The present invention is applied to recovering chemical energy oforganic matter in the form of a hydrogen gas and converting hydrogen gasto electric energy with high efficiency, and is applicable to a systemfor producing a hydrogen gas or a hydrogen-containing gas from ananaerobic digestion gas yielded by methane fermentation of organicwastes, and a system for generating electricity by supplying theproduced hydrogen gas or hydrogen-containing gas to a fuel cell.

1. A fuel cell power generation method utilizing methane fermentation oforganic matter, said method comprising: a methane fermentation step forsubjecting organic matter to methane fermentation to yield a digestiongas; a pretreatment step for pretreating the digestion gas yielded insaid methane fermentation step; a hydrogen production step for producinga hydrogen-containing gas from the pretreated gas in said pretreatmentstep; and a fuel cell power generation step for supplying thehydrogen-containing gas produced in said hydrogen production step to afuel cell to generate electricity; wherein said pretreatment stepcomprises an alkaline absorption step for absorbing carbon dioxide orcarbon dioxide and hydrogen sulfide contained in the digestion gasyielded in said methane fermentation step into an alkaline absorbentsolution to separate carbon dioxide or carbon dioxide and hydrogensulfide from the digestion gas.
 2. A method according to claim 1,wherein, in said alkaline absorption step, the digestion gas is broughtinto counterflow contact with the alkaline absorbent solution to absorbcarbon dioxide or carbon dioxide and hydrogen sulfide into the alkalineabsorbent solution to enrich the concentration of methane; and furthercomprising: a heat exchange step for heating the alkaline absorbentsolution which has absorbed carbon dioxide or carbon dioxide andhydrogen sulfide therein in said alkaline absorption step, by waste heatgenerated in said hydrogen production step and/or said fuel cell powergeneration step; and a regeneration step for regenerating the alkalineabsorbent solution by bringing the alkaline absorbent solution heated insaid heat exchange step into counterflow contact with a cathode vent gasdischarged from said fuel cell power generation step or a combustionexhaust gas discharged from said hydrogen production step to stripcarbon dioxide or carbon dioxide and hydrogen sulfide from the alkalineabsorbent solution.
 3. A method according to claim 2, wherein thetemperature of the alkaline absorbent solution prior to regenerating insaid regeneration step is lower than the temperature of the outlet ofstack cooling water from said fuel cell power generation step by 10° C.or less, while the temperature of the alkaline absorbent solution priorto absorbing in said alkaline absorption step is lower than thetemperature of the alkaline absorption solution prior to regenerating inthe range of 10° C. to 35° C.
 4. A method according to claim 2, whereinthe temperature of the alkaline absorbent solution prior to regeneratingin said regeneration step is lower than the temperature of the outlet ofstack cooling water from said fuel cell power generation step by 2° C.or less, while the temperature of the alkaline absorbent solution priorto absorbing in said alkaline absorption step is lower than thetemperature of the alkaline absorption solution prior to regenerating inthe range of 10° C. to 35° C. and is higher than the temperature of amethane fermentation liquid in said methane fermentation step by 2° C.or more.
 5. A method according to any one of claims 1 to 4, wherein saidalkaline absorption step comprises the step of bringing the gas afterthe absorption into counterflow contact with water to wash awayabsorbent solution droplets carried over into the gas.
 6. A methodaccording to any one of claims 1 to 5, further comprising adesulfurization step wherein the digestion gas yielded in said methanefermentation step is brought into counterflow contact with adesulfurizing solution containing an alkali chemical and a water-solubleoxidizing agent to absorb and oxidatively decompose hydrogen sulfide,and is then supplied to said alkaline absorption step.
 7. A methodaccording to any one of claims 1 to 6, wherein said hydrogen productionstep comprises a reforming step and a carbon monoxide shift reactionstep.
 8. A method according to claim 7, wherein said hydrogen productionstep further comprises a carbon monoxide selective oxidation step aftersaid reforming step and said carbon monoxide shift reaction step.
 9. Amethod according to any one of claims 1 to 8, wherein said fuel cellcomprises a proton exchange membrane fuel cell or a phosphoric acid fuelcell.
 10. A method according to any one of claims 1 to 6, wherein saidalkaline absorption step includes a controller for controlling theconcentration of methane in the gas after the absorption operation to acertain value.
 11. A method according to any one of claims 2 to 5,wherein a biodesulfurization step is provided and an off-gas dischargedfrom said regeneration step is led to said biodesulfurization step tobiodegrade hydrogen sulfide contained in the off-gas.
 12. A fuel cellpower generation system in which a digestion gas is yielded by methanefermentation of organic matter, and a hydrogen-containing gas isproduced from the yielded digestion gas and then supplied to a fuel cellto generate electricity, said system comprising: an alkaline absorptionapparatus for absorbing carbon dioxide or carbon dioxide and hydrogensulfide contained in the digestion gas yielded in the methanefermentation into an alkaline absorbent solution to separate carbondioxide or carbon dioxide and hydrogen sulfide from the digestion gasfor thereby enriching the concentration of methane in the gas.
 13. Asystem according to claim 12, further comprising: a heat exchanger forheating the alkaline absorbent solution which has absorbed carbondioxide or carbon dioxide and hydrogen sulfide therein in said alkalineabsorption apparatus, by waste heat generated in the production of thehydrogen-containing gas and/or in the power generation using thehydrogen-containing gas; and a regeneration apparatus for regeneratingthe alkaline absorbent solution by bringing the alkaline absorbentsolution heated in said heat exchanger into counterflow contact with acathode vent gas from said fuel cell or a combustion exhaust gas fromthe production of the hydrogen-containing gas to strip carbon dioxide orcarbon dioxide and hydrogen sulfide from the alkaline absorbentsolution.
 14. A system according to claim 13, wherein the temperature ofthe alkaline absorbent solution prior to regenerating in said alkalineregeneration apparatus is lower than the temperature of the outlet ofstack cooling water from said fuel cell using the hydrogen-containinggas by 10° C. or less, while the temperature of the alkaline absorbentsolution prior to absorbing in said alkaline absorption apparatus islower than the temperature of the alkaline absorption solution prior toregenerating in the range of 10° C. to 35° C.
 15. A system according toclaim 13, wherein the temperature of the alkaline absorbent solutionprior to regenerating in said alkaline regeneration apparatus is lowerthan the temperature of the outlet of stack cooling water from said fuelcell using the hydrogen-containing gas by 2° C. or less, while thetemperature of the alkaline absorbent solution prior to absorbing insaid alkaline absorption apparatus is lower than the temperature of thealkaline absorption solution prior to regenerating in the range of 10°C. to 35° C. and is higher than the temperature of a methanefermentation liquid in said methane fermentation by 2° C. or more.
 16. Asystem according to any one of claims 12 to 15, further comprising arinsing apparatus for bringing the gas after the absorption in saidalkaline absorption apparatus into counterflow contact with water towash away absorbent solution droplets carried over into the gas.
 17. Asystem according to any one of claims 12 to 16, wherein the digestiongas yielded in the methane fermentation is brought into counterflowcontact with a desulfurizing solution containing an alkali chemical anda water-soluble oxidizing agent to absorb and oxidatively decomposehydrogen sulfide, and is then supplied to said alkaline absorptionapparatus.
 18. A system according to any one of claims 12 to 17, whereinsaid fuel cell comprises a proton exchange membrane fuel cell or aphosphoric acid fuel cell.